By fluorination, it is possible to establish a very wide range of surface properties:                improvement of the barrier effect, build-up of permeation barrier layer/odour barrier;        improvement of the chemical resistance and the thermal stability;        activation for adhesion promotion for coating materials (paints, finishes, adhesives, etc.);        chemical roughening of the surfaces, microstructuring;        chemical smoothing of the surfaces, minimization of interfacial interactions;        reduction of the coefficients of friction, for example improvement of the mounting of seals;        permanent hydrophilic treatment of plastic surfaces, optimization of the wettability;        permanent hydrophobic treatment of plastic surfaces, reduction of water absorption.        
The treatment of non-polar polymer materials with fluorine results in a polar surface owing to partial substitution of the CH bonds by CF groups. The interactions of the surface thus produced with non-polar liquids are substantially reduced with regard to wettability and swellability. Finally, the chemical composition of the surface results in a reduction of permeation.
Exchanging hydrogen atoms for fluorine atoms results in dipoles in the polymer. As a result, the basic precondition for chemical bonding is created. In addition, the incorporation of fluorine atoms into the polymer building blocks produces chemical roughening of the surface, with the result that mechanical anchoring of the coating material is achieved. The combination of chemical bonding and mechanical anchoring permits a strong adhesive bond that is stable in the long term.
By incorporating fluorine atoms into the polymer, a “Teflon-analogous” surface structure is formed. A property jump associated therewith results in an improvement in the thermal stability and the chemical resistance of the plastic, for example to acids and alkalis.
Two different methods are used for carrying out the fluorination.
The in-line method can be applied only to hollow bodies. In this method, the fluorination—of only the inner surface of the hollow body—takes place during manufacture in the blow extrusion method. In contrast to the off-line method, an additional treatment effort is dispensed with thereby.
The off-line method can be applied both to hollow bodies and to profiles and films of plastic. It is a method for aftertreatment of the finished parts in a reaction chamber. The total surface is treated thereby. The advantage of the off-line method consists in the high flexibility of the process sequences. By decoupling the treatment from the manufacture, it is possible to vary the parameters of the method within a wide range so that optimum adaptation of the fluorination to the general conditions of manufacture can be effected.
The advantages of the fluorination method are, inter alia:                High and uniform distribution of the surface tension (polarity), independently of the geometry of the parts.        No damage to the base polymer.        Long-term stability of the treatment effect.        Bulk treatment of small parts.        Applicable to thin-walled components.        Flexible process sequences.        High degree of automation.        High process reliability.        
The fluorination of different plastics for producing a fluorine-containing layer on these plastics is widely used. The fluorine-containing layer on the plastics has a certain barrier effect with respect to the penetration of volatile substances. For example, the plastic tank walls of fuel tanks are fluorinated for preventing the permeation of fuels for vehicles.
EP 1 234 849 A1 discloses a process for the treatment of plastic surfaces by gas-phase fluorination, in which a reaction chamber with the material to be treated, which has a plastic surface, is evacuated to a pressure of not more than 10 mbar absolute or a tenth of the treatment pressure or less and a fluorine-containing gas mixture of defined composition is introduced into the reaction chamber until a treatment pressure in the range from 25 to 250 mbar absolute is reached.
EP 0 566 883 B1 describes a process for the fluorination of inner surfaces of plastic containers with a gas which contains, for example, 1% of fluorine, at a pressure of 2 to 10 bar and a surface temperature of 60 to 250° C. of the inner surface of the hollow body. The fluorine-containing treatment gas enters the hollow body at a temperature between −20° C. and −196° C.
Furthermore, EP 0 809 670 B1 discloses a fluorination process which takes place in four process steps at different pressures. In the first stage of the fluorination process, the ambient pressure P1 of the polymer to be fluorinated is reduced to 0.001 to 0.5 kPa with the aid of a vacuum pump in the absence of the fluorinating agent in the reaction chamber. In a second stage, the pressure P2 in the reaction chamber is increased to 0.1 to 300 kPa with the aid of a fluorinating agent. The pressure in the reaction chamber is then reduced to 0.01 to 1 kPa. In a third stage, the pressure is adjusted to 0.1 to 300 kPa. Thereafter, the pressure is reduced again to 0.01 to 1 kPa. In the fourth stage, the pressure is adjusted to 0.1 to 1000 kPa. Thereafter, the fluorinated polymeric material is removed from the reaction chamber. This process is complicated since four stages are envisaged altogether and the pressure is also reduced in each case between the individual stages and requires at least one vacuum pump for the respective pressure reductions between the individual stages.
Furthermore, DE 198 32 559 C2 discloses a fluorination process for plastics in which a plastic article to be fluorinated is introduced into a reaction chamber and, after evacuation, is subjected to the fluorination in an atmosphere containing an inert gas and fluorine. The gas present in the reaction chamber and the article to be fluorinated are kept at a temperature greater than or equal to 40° C. during the evacuation and the fluorination and, after the fluorination, a vacuum is applied and is maintained until unconverted fluorine molecules have diffused out of the plastic article. The pressure of the fluorine/nitrogen mixture during the fluorination is 40 mbar.
A common feature of the known fluorination processes is the treatment of the plastic in a vacuum-tight reaction chamber and the fluorination under reduced pressure of the fluorination gas mixture. The plastic parts or plastic mouldings to be treated are introduced into the vacuum-tight reaction chamber and the latter is evacuated for removing atmospheric oxygen and is flooded with an inert gas. After further to evacuation of the reaction chamber, the fluorine-containing treatment gas is metered into the reaction chamber. The fluorination takes place either continuously at reduced pressure or in a plurality of stages, the individual stages taking place at different pressures and evacuation being effected again in each case between the stages. In the prior art, the reaction chambers are flushed by repeated evacuation and flooding with inert gas after the end of the fluorination. The complexity of the apparatus in the known processes is high since the reaction chambers have to be vacuum-tight and at least one vacuum pump has to be present for the evacuation.
The fluorination in vacuo of pressure-sensitive plastic surfaces which have, for example, a closed thin-skinned honeycomb structure and of closed-pore plastic foams faces in practice the problem that, in vacuo, the thin-skinned honeycomb structure and the pores of the plastic foams burst open since the gases enclosed therein, such as air, nucleating agents, or blowing agents, are generally gaseous, expand in vacuo and the structure of such plastics is thus damaged.